Generally as you have read the OPG on the MK3/3.5 MX-5 is really a cosmetic thing, you should treat it as part of the In Car Entertainment system rather than any real read out of oilpressure. As I understand it the reading comes from a linear (On/Off) OilPressure Switch combined with a signal from the ECU rather than an OilPressure Transducer. Basically as long as it is registering something you shouldn't worry, as usual regular checks of the dip stick should not be ignored.